1. Field of the Invention
The present invention generally relates to image forming apparatuses and, more particularly, to an image forming apparatus such as a digital copy machine or a laser printer, which forms a visible image using an electrophotography process.
2. Description of the Related Art
In an image forming apparatus of a electrophotograph system using a so-called negative/positive process system which forms a toner image by attaching a toner onto a part of a photosensitive member irradiated by a laser beam, a so-called overlap scanning method is known as a method for doubling a pixel density in a sub-scanning direction in which a surface of a recording medium is scanned by a light beam having a beam diameter larger than a sub-scanning pitch so as to form an image in a position where a plurality of beams overlap with each other.
For example, the integrated circuit (IC), designated as PM-2060i, manufactured by Oak Technology in the United States, has a function to convert an input of binary image data of 1200 dpi into a multi-value PWM output of a write cycle of 600 dpi, and capable of outputting a print corresponding to 1200 dpi without optically or mechanically changing a 600 dpi print engine.
Moreover, a so-called multi-beam scanning system which carries out an exposure scan of a surface of a photo conductor by a plurality of light beams simultaneously is widely used for the purpose of improvement in record density or improvement in recording speed.
A description will be given, with reference to FIG. 1, of an example of operation of the write-in signal processing part of an exposure device in an image forming apparatus using a multi-beam scanning system. A description will be given below of a case in which an image having 1200 dpi record density in both the main scanning direction and sub scanning direction by using an image forming apparatus of 600 dpi record density in both the main scanning direction and the sub scanning direction.
In FIG. 1, a line memory 101 stores 1200 dpi image data corresponding three lines, and sends the image data corresponding to a recording position in a main scanning direction.
The pattern detection processing circuit 102 sends a code corresponding to a matrix pattern to a look-up table (LUT) circuit 103 with reference to ON-OFF information of a total of six dots consisting of two consecutive dots to be currently recorded in the main scanning direction and four dots positioned above and below the two consecutive dots. The 2×3 dot matrix, two in the main scanning direction and three in the sub scanning direction, is referred to as a pattern detection matrix.
The LUT circuit 103 sends predetermined pulse width signal and pulse position signal to a pulse width modulation (PWM) circuit 104 in response to the code sent from the pattern detection processing circuit 102.
The PWM circuit 104 sends a PWM signal to a laser diode (LD) driver 105 based on the received pulse width signal and pulse position signal. The PWM signal is output in synchronization with a video clock signal (frequency of 600 dpi).
The LD driver 105 supplies to a laser diode (LD) 106 a drive current upon receipt of an “ON” signal and an offset current upon receipt of an “OFF” signal.
In the above-mentioned structure, the LD 106 emits a light beam so as to perform exposure scan on the surface of the photoconductor drum (not shown) by the light beam. The pitch of the exposure scan in the sub scanning direction is smaller than a beam diameter, which is defined by a value at which an intensity of light is 1/e2 of an intensity of light at the center of the beam. An image formation can be performed with a smaller pitch than the pitch of the exposure scan in the sub scanning direction by forming an electrostatic latent image also at a position on the photoconductor drum where adjacent light beams overlap with each other.
For example, it is supposed that the image data and the pattern detection matrix are as shown in FIG. 2. In the figure, x0, x1, . . . and y0, y1, . . . indicate scan positions at 600 dpi in the main scanning direction and the sub scanning direction, respectively. The PWM circuit 104 outputs the PWM signal at the same position in the main scanning direction over two lines, and outputs the PWM signal of the set-up pulse width. Thus, an image is formed in an area where areas irradiated by the light beams overlaps with each other. In this way, the formation of 1200 dpi image can be achieved by using the image forming apparatus of 600 dpi record density without changing a video clock frequency or a feed speed in the sub scanning direction.
As mentioned above, it is necessary to set the pulse width and the pulse position corresponding to the code sent from the pattern detection processing circuit 102 to the LUT circuit 103. The important matter at the time of setting up the pulse width is the following three points.
(1) When a solid black image is output, a maximum pulse width must be set so that a solid black portion has a proper concentration.
(2) When a plurality of single dot width horizontal lines offset by 1200 dpi to each other are output, a pulse width with respect to a corresponding code must be set so that a horizontal line formed in the scan position of the light beam and a horizontal line formed between two scan positions are in the same concentration.
(3) When a gray scale, which consists of a halftone pattern in which ON pixel density varies continuously, is output, a pulse width corresponding to each code must be set so that the variation in the concentration in the gray scale is continuous.
However, when an image is output using a look-up table prepared in consideration of the above-mentioned points, the following problems may occur.
For example, when image data consisting of a set of halftone patters as shown in FIGS. 3A and 3B are output, there may be a case in which the output images differ in concentration from each other although the density of ON pixels is equal. This is because the code generated by the pattern detection processing circuit 102 are different from each other between the two patterns since positions of the halftone patterns in the main scanning direction are offset from each other by a single dot of 1200 dpi.
In Japanese Patent Application No. 2002-27110 filed by the present applicant, the above-mentioned problems are solved by outputting a plurality of test patterns having the same ON pixel density but different relative positions with respect to the pattern detection matrix and setting a pulse width corresponding to each code in all levels so as to equalize the concentrations of the test patterns. However, it is not efficient to set the pulse width by a comparison of concentration for all codes generated by the pattern detection processing circuit 102.
In order to solve the above-mentioned problem, Japanese Patent Application No. 2001-107475 filed by the present applicants suggests a simplified procedure for setting the pulse width. With respect to three levels from among the levels to be set up, a plurality of test patterns having the same ON pixel density but having different positions with respect to the pattern detection matrix are output so as to set the pulse width corresponding to each code so that concentrations of the test patterns are equalized to each other. The remaining levels are determined by interpolation using the values of the levels that has already been set.
However, when a look-up table is prepared according to the above-mentioned method, fault may occur in an output image. For example, when a look-up table was prepared according to the above-mentioned characteristic and test pattern images are output by an image forming apparatus having a certain characteristic of exposure amount versus output image concentration, concentrations of the test pattern images, which have the same ON pixel density but different relative positions with respect to the pattern detection matrix, were equal to each other. However, when a look-up table was prepared according to the above-mentioned characteristic and test pattern images are output by an image forming apparatus having a different characteristic of exposure amount versus output image concentration, concentrations of the test pattern images, which have the same ON pixel density but different relative positions with respect to the pattern detection matrix, were different from each other. This is because pulse width determined by interpolation using the values of levels that have been determined were not appropriate.
In order to solve this problem, Japanese Patent Application No. 2001-183589 filed by the present applicants suggests that a setting is made so that concentrations of test patterns formed have the same ON pixel density. In the technique disclosed in this patent application, a plurality of test patterns are output with respect to at least three levels from among levels to be setup, the test patters being image patterns having the same ON pixel density but having different relative positions with respect to pattern detection matrix so as to set the concentrations of the thus-formed test patterns having the same ON pixel density to be equal to each other. With respect to the pulse width of remaining levels, a temporary value is determined by interpolation based on the values of the pulse widths corresponding to at least three levels that have been already set so as to output a plurality of test patterns having the same ON pixel density but having different relative positions with respect to the pattern detection matrix.